Low quality mp4 renders
You should increase the bitrate. It determines the compression of the video data.
A starting point could be to set the bitrate as least as high as in the input file, and then add the Mp3 rate of 192kbs.

1920 x 1080 = 2,073,600 pixel count 
24 frames per second 
4 = High Motion 

(2,073,600 x 24) x 4 x 0.07 = 13,934,592 bps / 1024 = 13,608 kbps bitrate


See sections 4 (Quality versus Streaming) and 5 (Starting Points), in this tutorial.